The present invention generally relates to aluminum nitride (AlN) powder slurries having a viscosity that favors processing of the slurries into ceramic greenware. The present invention also relates to the use of certain organic compounds to modify reactivity of aluminum nitride with organic components of the slurries and preclude thickening thereof to such an extent that they are not readily processible.
AlN exhibits certain physical properties that make it particularly suitable for use in a variety of applications. Some applications, such as those in packaging components for electronic circuitry, require substantially full theoretical density and high thermal conductivity. High quality AlN powder, when densified by sintering, hot-pressing or other suitable means, generally satisfies these requirements.
AlN powder is typically converted to ceramic greenware suitable for sintering by slip casting or tape casting a slurry of the powder in a suitable solvent. As an alternative, AlN powder can be recovered from the slurry by spray drying. After recovery, the AlN powder can be dry pressed, cold pressed, cold isostatic pressed or otherwise converted to greenware via conventional technology. Because AlN powder readily hydrolyzes or reacts with water, organic solvents are favored over water. In an effort to increase density of the greenware, organic lubricants are often added to a slurry of AlN powder and one or more sintering aids in an organic solvent. A principal reason for adding the lubricants is to enhance particle-particle sliding. Typical lubricants include stearic acid, oleic acid and aluminum stearate. Unfortunately, these lubricants tend to react with AlN surfaces and form reaction products that thicken the slurry to a consistency resembling that of pudding. Such a consistency is not readily processible in ceramic forming equipment.
One means of overcoming reactivity of AIN powder surfaces involves oversaturating said surfaces with the lubricant(s). Oversaturation may occur by doubling or even tripling what might otherwise be regarded as a "normal" amount of lubricant. Unfortunately, this also leads to other problems. One such problem is a phenomenon known as "springback" during forming. Springback, as used herein, means the elastic release of stored elastic energy to cause a dimensional increase upon release of a pressed ceramic body from a die. In other words, dimensions of the body after release from the die exceed those of the body prior to release from the die. This can, in turn, lead to delamination or separation of pressed material within the ceramic body. The delamination or material separation induces cracks within the ceramic body. A second problem is a high residual carbon content after organic compounds are "burned out" or removed by heating prior to sintering. A high residual carbon content can adversely affect physical properties of a resultant sintered body. Such physical properties include thermal conductivity, density and electrical performance.
It would be desirable if there were a simple, yet effective means or method of modifying the surfaces of AlN powder in an organic liquid slurry to minimize thickening of the slurry. It would also be desirable if there were a material that could be added to a thickened organic liquid slurry based upon aluminum nitride to reduce its viscosity to a level that equates to ready processibility.